1. Field of the Invention
The present invention relates to a method for transferring an electric charge from an image sensing device and an image sensing device preferable for use in a video camera enabling to take pictures of moving images and still images.
2. Description of the Related Art
A solid-sate image sensing device outputs an electric signal converted from an incoming light image and is utilized for a video camera and a digital still camera. As for the solid-state image sensing device, a high resolution image can be obtained by increasing a number of pixels of the image sensing device. Recently, video camera users want to obtain not only moving images but also a high resolution still image taken by a video camera. Accordingly, the image sensing device for video camera is in need of having more pixels.
FIG. 6 is a plan view of a conventional solid-state image sensing device. In FIG. 6, the image sensing device is composed of a photoelectric converter 31 for converting light images into an electric signal, a vertical transfer CCD (hereinafter referred to as “VCCD”) 32 for transferring an electric charge to vertical shift register, a horizontal transfer CCD (hereinafter referred to as “HCCD”) 33 for transferring an electric charge to horizontal shift register, an amplifier 34 for amplifying the electric charge from the HCCD 33.
A timing generator not shown generates a read-out pulse for each VD (vertical sync signal) period of a television signal. The electric charge stored in each photoelectric converter 31 is transferred to the adjacent VCCD 32 each time when generating the read-out pulse.
The VCCD 32 shifts the electric charge from each photoelectric converter 31 to the succeeding stage of the VCCD 32 one by one toward the HCCD 33. The HCCD 33 outputs all electric charges transferred from the VCCD 32 through the amplifier 34. Accordingly, the velocity of shifting the electric charge in HCCD 33 should be adjusted to the speed which can output all the electric charges transferred from the VCCD 32 through the amplifier 34 at each time when electric charges equivalent to one stage of the VCCD 32 are transferred from the VCCD 32.
As described above, the electric charge stored in each photoelectric converter 31 is temporally transferred to VCCD 32. The electric charges transferred from VCCD 32 to HCCD 33 are shifted to horizontal direction and outputted through the amplifier 34.
When the number of pixels in the vertical direction and the horizontal direction of photoelectric converter 31 increases, the number of stages for shifting electric charge in the VCCD 32 and the HCCD 33 also increases. Accordingly, a clock frequency for transferring electric charge should be increased as the number of pixels of the photoelectric converter 31 increases.
However, the increase of clock frequency may cause problems such as generating heat, increasing electric power consumption, and deteriorating signal to noise ratio. FIG. 7 shows an example of solid-state image sensing device of which clock frequency for transferring electric charge of HCCD is lowered.
In FIG. 7, the solid-state image sensing device is composed of a pixel area 41 for disposing a photoelectric converter in vertical and horizontal directions and constituting VCCD, HCCDs 42 and 43 for shifting electric charge transferred from VCCD in the pixel area 41, adders 44 and 45 for adding a reference signal of predetermined level to the electric charge to be outputted from HCCDs 42 and 43 respectively, amplifiers 46 and 47 for amplifying the reference signal and the electric charge supplied from the adders 44 and 45 respectively. The reference signal is utilized for compensating scatter of each channel characteristic before synthesizing electric charge of each channel.
The electric charge transferred from the VCCD in the pixel area 41 is dispersed into the HCCD 42 and the HCCD 43, and is shifted to the vertical direction by each of the HCCDs 42 and 43. Accordingly, the clock frequency necessary for shifting the electric charge in each of the HCCD 42 and 43 can be reduced to a half of the clock frequency for single HCCD.
In case of the solid-state image sensing device shown in FIG. 7, the pixel size generally becomes smaller when a number of pixels increases. If the pixel size becomes smaller, the transferring efficiency for electric charge becomes insufficient and resulted in that the quality of image may be affected.
Recently, the whole system of video camera including the optical system is miniaturized and the solid-state image sensing device is also miniaturized into ⅓ inch or ¼ inch size. However, when many photoelectric converters are disposed in a miniaturized image sensing device, the transferring efficiency becomes insufficient for VCCD to shift its electric charges to HCCD in the pixel area 41 and resulted in that the quality of image may be affected.